To test his idea, Mercer spent a night in the kitchen of his Waterloo home pouring his fruit, yoghurt and oats concoction into Tesco water bottles, on which he had Pritt Sticked his own labels. After a long night that killed off several blenders, he set up a trestle table in Waterloo and handed out his shakes to commuters for free, in exchange for their email addresses.

Later that day, he sent out surveys asking for feedback. “The positive responses gave me the impetus to quit my job,” Mercer says. That decision led to four months experimenting with recipes, and biking around London train stations looking at sites. “I would stand there for hours, counting the number of people who walked by and the proportion who stopped in coffee shops. On one occasion I was chucked out by police for loitering.”

Mercer worked out that he needed a footfall of about 10,000 people between 6.30 and 10am to make the business viable. Eventually he secured a first site – at Waterloo station – and spent £10,000 of savings on a van, stall and cooking premises under a Deptford railway arch.